A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. A transistor is used for a display device typified by a liquid crystal television. Silicon is known as a material for a semiconductor thin film applicable to a transistor; however, an oxide semiconductor has attracted attention in recent years.
As a material for the oxide semiconductor, zinc oxide or a material containing zinc oxide as its component is known (e.g., Patent Documents 1 and 2). Further, a transistor formed using an amorphous oxide (oxide semiconductor) having an electron carrier concentration of less than 1018 cm−3 is disclosed (e.g., Patent Document 3).